(a) Field of the Invention
The present invention relates to RF switches and is useful in particular, but not exclusively, for RF switches intended for use in space.
(b) Description of Related Prior Art
It is known that RF switches employed in space, where they are required to operate in a vacuum or a near vacuum, are subjected to special problems, including multipactor discharge, corona discharge and elevated temperatures.
Multipactor discharge is a resonant RF discharge which is sustained by the emission of secondary electrons from discharging surfaces. It is known (see P. F. Clancey, "Multipactor Control in Microwave Space Systems", Microwave Journal, Volume 21, March 1978, pp. 77-83) that three conditions are required to produce multipactor discharge, these conditions being (1) that the multipacting gap must in vacuum; (2) that the secondary electron emission coefficient of the surfaces, which depends on the type of surfaces, their cleanliness and their electron energy, must be greater than unity; and (3) that the RF power, the gap spacing and the frequency must be such as to ensure resonance between the electron motion and the field.
The corona discharge is also determined by the spacing, the signal frequency, and the power, and in addition requires a low level vacuum, and is unlikely to occur in a switch provided with good venting and employing materials that do not out-gas significantly, even at elevated temperatures.
Switches are also required to dissipate heat in order to avoid elevating internal parts of the switches.
It has previously been proposed to employ, as a high power switch for space use, a switch employing a metallic reed forming a transmission line between two points to be connected (see U.S. Pat. No. 4,317,972, issued Mar. 2, 1982 to Evert Kjellberg). The metallic reed provided in this prior art switch is activated by means of solenoids and is surrounded by a dielectric material. It has been found, however, that such a switch tends to be unreliable in use, apparently as a result of multipactor discharge and poor thermal conductivity.